As such a storage device, a solid-state drive (SSD) including a nonvolatile semiconductor memory such as a flash memory is known. In line with an increase in the capacity of a flash memory, SSDs are used in place of conventional hard disk drives (HDDs). In the flash memory, a floating gate covered by an insulating film is formed in a memory cell transistor. Data is stored by applying a potential to the floating gate. The threshold voltage of the memory cell transistor is changed depending on the presence or absence of gate potential. By this change, the stored data is read. However, repetition of data erasing and writing leads to deterioration of the insulating film which covers the floating gate (the combination of erasing and writing may be referred to as rewriting). The potential applied to the floating gate is lost over time. Ultimately, no potential remains. Thus, correct data cannot be read. In other words, the stored data is lost.
To solve this problem, in the SSD, a refresh process is performed. In the refresh process, the stored data is read from the memory and is rewritten before the data is lost. For example, the refresh process is performed when the power source is switched on, or the refresh process is performed in predetermined regular time intervals while the power source is in an on-state. Potential is applied again to the floating gate by the refresh process. Thus, the loss of the stored data is prevented. However, since the refresh process requires reading and rewriting, reading and writing of the host is suspended during the refresh process. Thus, the reading and writing performance of the host is degraded.